This invention relates to a process for the beneficiation of solid carbonaceous fuel materials and more particularly to a process for the beneficiation of oxidized coal.
Known resources of coal and other solid carbonaceous fuel materials are far greater than the known resources of petroleum and natural gas combined. Despite this enormous abundance of coal and related solid carbonaceous materials, reliance on these resources, particularly coal, as primary sources of energy, has been, for the most part, discouraged. The availability of cheaper, cleaner burning, more readily retrievable and transportable fuels, such as petroleum and natural gas, has, in the past, cast coal to a largely supporting role in the energy field.
Current world events, however, have forced a new awareness of global energy requirements and of the availability of those resources which will adequately meet these needs. The realization that reserves of petroleum and natural gas are being rapidly depleted in conjunction with skyrocketing petroleum and natural gas prices and the unrest in the regions of the world which contain the largest quantities of these resources, has sparked a new interest in the utilization of solid carbonaceous materials, particularly coal, as primary energy sources.
As a result, enormous efforts are being extended to make coal and related solid carbonaceous materials equivalent or better sources of energy, than petroleum or natural gas. In the case of coal, for example, much of this effort is directed to overcome the environmental problems associated with its production, transportation and combustion. For example, health and safety hazards associated with coal mining have been significantly reduced with the onset of new legislation governing coal mining. Furthermore, numerous techniques have been explored and developed to make coal cleaner burning, more suitable for burning and more readily transportable.
Gasification and liquefaction of coal are two such known techniques. Detailed descriptions of various coal gasification and liquefaction processes may be found, for example, in the Encyclopedia of Chemical Technology, Kirk-Othmer, Third Edition (1980) Volume 11, pages 410-422 and 449-473. Typically these techniques, however, require high energy input, as well as the utilization of high temperature and high pressure equipment, thereby reducing their widespread feasibility and value.
Processes to make coal more readily liquefiable have also been developed. One such process is disclosed in U.S. Pat. No. 4,033,852 (Horowitz et al.). This process involves chemically modifying the surface of the coal, the effect of which renders a portion of the coal more readily liquefiable than the natural forms of coal.
In addition to gasification and liquefaction, other methods for converting coal to more convenient forms for burning and transporting are also known. For example, the preparation of coal-oil and coal-aqueous mixtures are described in the literature. Such liquid coal mixtures offer considerable advantages. In addition to being more readily transportable than solid dry coal, they are more easily storable, and less subject to the risks of explosion by spontaneous ignition. Moreover, providing coal in a fluid form makes it feasible for burning in conventional apparatus used for burning fuel oil. Such a capability can greatly facilitate the transition from fuel oil to coal as a primary energy source. Typically, coal-oil and coal-aqueous mixtures and their preparation are disclosed in U.S. Pat. Nos. 3,762,887, 3,617,095 and 4,217,109 and British Pat. No. 1,523,193.
Regardless of the form in which the coal is ultimately employed, the coal must be cleaned because it contains substantial amounts of sulfur, nitrogen compounds, and mineral matter, including significant quantities of metal impurities. During combustion these materials enter the environment as sulfur dioxides, nitrogen oxides and compounds of metal impurities. If coal is to be accepted as a primary energy source, it must be cleaned to prevent pollution of the environment, either by cleaning the combustion products or the coal prior to burning.
Accordingly, physical, as well as chemical, coal cleaning (beneficiation) processes have been extensively explored. In general, physical coal cleaning processes involve grinding the coal to release the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. However, because the costs of preparing the coal rise exponentially with the amount of fines, there is an economic optimum in size reduction. Moreover, grinding coal even to the finest sizes is not effective in removing all impurities.
Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are divided into four general categories: gravity, flotation, magnetic and electrical methods.
In contrast to physical coal cleaning, chemical coal cleaning techniques are in a very early stage of development. Known chemical coal cleaning techniques include, for example, oxidative desulfurization of coal (sulfur is converted to a water-soluble form by air oxidation), ferric salt leaching (oxidation of pyritic sulfur with ferric sulfate), and hydrogen peroxide-sulfuric acid leaching. Other methods are disclosed in the above-noted reference to the Encyclopedia of Chemical Technology, Volume 6, pages 314-322.
A recent promising development in the art of chemical coal beneficiation is disclosed in U.S. Pat. No. 4,304,573 incorporated herein by reference. In summary, according to this coal beneficiation process, coal is first cleaned of rock and the like and pulverized to a fine size. The pulverized coal, now in the form of a water slurry, is then contacted with a mixture comprising a polymerizable monomer, polymerization catalysts and fuel oil. The resultant surface treated coal is highly hydrophobic and oleophilic and is thus readily separated from unwanted ash and sulfur using oil and water separation techniques. Moreover, the hydrophobic coal can be readily further dehydrated to very low water levels without employing costly thermal energy. The clean, very low moisture content coal, resulting from this process can then be employed as is, i.e., a dry solid product, or used to form advantageous coal-oil or coal-aqueous mixtures.
However, all coal does not respond equivalently to beneficiation procedures. For example, as a result of the varient chemical make-up of the known classes of coal, e.g. lignite, anthracite, bituminous, etc., each class responds differently to beneficiation. The so-called low rank coals, i.e. low rank bituminous, lignite and peat, contain water of hydration, which impairs and at times prevents, beneficiation by conventional froth flotation processes. For example, these coals do not respond satisfactorily to the so-called Otiska process.
Furthermore, coals, in general, upon exposure to air and varying amounts of water, become "oxidized" i.e. have oxidized surfaces. Such oxidized coal is characterized by changes in wettability and floatability as related to recovery by froth flotation procedures. The floatability of coals is gradually decreased by the increase in the extent of oxidation. As a result, the recoveries of beneficiated coal become significantly reduced.
Previous attempts to overcome the detrimental effects of oxidized coal in froth flotation have been essentially of a chemical nature. Most have had only limited success. It is, therefore, highly desirable to provide a process for altering or conditioning the surfaces of oxidized coals in order to achieve higher recoveries of the beneficiated product.